I've logged bug 444 (http://www.w3.org/Bugs/Public/show_bug.cgi?id=444)
to the DOM TS to address the issues that were raised in this thread.
The existing L2 Core tests elementsetattributens01 and 03 appeared to
cover using a non-null namespace and a non-prefixed qualified name in
memory on a call to Element.setAttributeNS. I added DOMWriterTest5 and
6 to check serialization of attributes added in this manner.